You can't buy games from one region in another. It doesn't matter where your Steam account is from, if you're connecting to your Brazillian account on the US Steam Store (because you have an American IP because you're in the united states), you'll access the US store and you won't be able to buy anything (because your account says you're from one country while you're accessing from another country).
You'd have, at that point, contact Steam support and let them know that you're traveling. Your account will then be unlocked to buy games FROM THE REGION you're traveling to.
In other words, while in the US, you'll be able to buy games from the US store, as long as you notify Steam support. NOT the Brazilian store.